One of the quietly clever moments of "World War Z" comes early, when the hero, played by Brad Pitt, battles his way through a zombie onslaught in the worst apartment building stairwell in Newark. He emerges onto the roof and, as his family watches, runs to the edge, half-leaning off the high-rise, and counts to twelve, explaining to them that he got some zombie blood in his mouth. If he's going to turn "Zeke," as soldiers come to nickname the zombies, he knows it's going to happen in a matter of seconds, and he wants to have an exit handy.

The zombies in "World War Z" are some of the most ferocious the genre has ever seen: full-sprint fast, hungry, heedless and instantly contagious. No wait-and-see, keep-him-under-observation — the line from terrified human to terrorizing zombie is no wider than the line between life and death itself. That speed of transmission is what makes "World War Z" perhaps the first zombie movie that's more epidemiological than supernatural. Pitt's superdad is a retired United Nations expert in conflict zones, and as he tries to suss out the solution to the world zombie epidemic, he's more interested in causes and quirks, less in the military response.

It's a tack that works because the zombies are too much for the military to handle. All the militaries, in fact. This is a divergence of degree from the source material, Max Brooks' 2006 novel, which did detail the political and military solutions to the zombie plague. (Spoiler: Many zombie heads are crushed with heavy implements.) But from these newer, even more overwhelming zombies are those mind-popping scenes of zombies sluicing over walls and careening through narrow streets — zombies as locust swarm, zombies as avalanche, zombies as wildfire. For as brainy as "World War Z" is at times, it's most successful when it confronts you with this raw unnatural force of nature. The bloat of the $190 million reported budget for this film is evident in the digital effects, which at times resemble a hellish version of a "Where's Waldo?" tableau.

If not in its particulars (or even in its characters) the film at least stays true to the most chilling theme of the book: the devastation wrought by panic. We see explosions and wrecks and general chaos well ahead of the zombies themselves, and when everyone eats by looting and thieving, only the looters and thieves will survive. When Pitt tries to explain this to a Spanish-speaking family, he reduces it to the simple phrase "movement is life." Just as the undead keep running, so must everyone else.

Director Marc Forster ("Quantum of Solace," "Finding Neverland") keeps the super zombie freak-out close-ups to a relative minimum. Less than the fear of a single zombie attack is the totality of the worldwide devastation. Josef Stalin is credited with the evil quip that one dead is a tragedy, where 20 million dead is a statistic. Maybe so, but the looming possibility of 7 billion undead is one scary stat. Pitt gets to carry the one-man burden; his wife (Mireille Enos) and kids are safe on an aircraft carrier just so long as he's out chasing the mystery of how anyone is going to survive. His world shrinks as the catastrophe grows. Like news, pandemics are ultimately local. It's all a crackling ride, in sum. But if you're not quite moved by the family story at the center of "World War Z," that's only natural. In its version of events, you were probably eaten alive by your own family days earlier.